This invention relates to objective lenses for use with digital cameras employing charged coupled devices (CCDs) or similar light sensitive electronic components. Objective lenses for such applications are described in various references, such as Rose et al., xe2x80x9cPhysical Limits to the Performance of Imaging Objective lens,xe2x80x9d Physics Today, September 1989, pages 24-32 and the references cited therein; and in Sequin et al., xe2x80x9cCharge Transfer Devices,xe2x80x9d Advances in Electronics and Electron Physics, suppl. 8, L. Marton editor, Academic Press, New York, 1975, the relevant portions of all of which are incorporated herein by reference.
Digital cameras use objective lenses to form images on the electronic imaging sensors such as CCD or CMOS imagers. For hand-held devices such as a cellphone, it is required that the digital camera imaging module be small. This requires that the objective lens have very compact size or height or low-profile.
Low profile objective lenses have relatively small lens elements which if they are too small, can be difficult to handle and assemble into a finished unit. Low cost is an important consideration for lenses for electronic imaging, especially where the objective lens is to be part of a mass marketed product. In addition, the electronic imagers used with such devices are improving in terms of image quality. The use of small high quality lenses increases the cost of assembly. The present invention is believed to provide a means of achieving a high level of optical performance with a minimum number of lens elements, thereby keeping the cost down, and a lens height suitable for use in electronic cameras and cell phones.
U.S. Pat. No. 6,097,551 to Kreitzer describes an objective lens design with two elements. The first element is specifically characterized as having a negative optical power while the second element is described as having a positive optical power. The arrangement of lenses in the design of the xe2x80x98551xe2x80x99 patent to Kreitzer tend to produce an objective lens design with a high profile making the arrangement of the xe2x80x98551xe2x80x99 patent unsuitable for compact cameras.
U.S. Pat. No. 6,449,105 to Dou describes an objective lens with two elements. However, as in the case of the xe2x80x98551xe2x80x99 patent, the design of the xe2x80x98105xe2x80x99 patent also requires that the first element have a negative optical power. The arrangement of the lens elements shown in the design of the xe2x80x98105xe2x80x99 patent, therefore also tend to form an objective lens that has a high profile thereby making the design of the xe2x80x98105xe2x80x99 patent unsuitable for compact cameras.
In U.S. Pat. No. 6,441,971, the same inventor describes a three-element objective lens. The first element is shown as a positive meniscus. The three element lens of the xe2x80x98971xe2x80x99 patent has a higher profile than the present invention and because of its increased component count, it is more costly to manufacture.
It is therefore an object of the present invention to provide an objective lens for electronic imaging which:
1. uses only two lens elements to minimize costs;
2. has a very short height for a given imager size; and
3. has a level of optical performance compatible with that of modern imagers.
To achieve these objects and other objects, the invention provides a two element objective lens wherein a first lens element 20, located on the object side of the objective lens has a positive optical power and is shaped as a meniscus element. The first lens element has a convex surface that faces the object. A second lens element 28, is located on the image side of the objective lens. The second lens element has a first lens surface facing the second surface of the first lens. The second surface of the second lens faces an image plane. The second surface of the second lens is aspheric.